Lithographic apparatus and device manufacturing method

ABSTRACT

In a lithographic projection apparatus, a structure surrounds a space between the projection system and a substrate table of the lithographic projection apparatus. A gas seal is formed between said structure and the surface of said substrate to contain liquid in the space.

This application is a continuation of U.S. patent application Ser. No.14/743,775, filed Jun. 18, 2015, now allowed, which is a continuation ofU.S. patent application Ser. No. 13/722,830, filed Dec. 20, 2012 (nowU.S. Pat. No. 9,091,940), which is a continuation of U.S. patentapplication Ser. No. 13/149,404, filed May 31, 2011 (now U.S. Pat. No.8,797,503), which is a continuation of U.S. patent application Ser. No.12/153,276, filed May 15, 2008 (now U.S. Pat. No. 7,982,850), which is acontinuation of U.S. patent application Ser. No. 11/239,493, filed Sep.30, 2005 (now U.S. Pat. No. 7,388,648), which is a continuation of U.S.patent application Ser. No. 10/705,783, filed Nov. 12, 2003 (now U.S.Pat. No. 6,952,253), which claims priority from European patentapplications EP 02257822.3, filed Nov. 12, 2002, and EP 03252955.4,filed May 13, 2003, all of the above-referenced applications hereinincorporated in their entirety by reference.

FIELD

The present invention relates to immersion lithography.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to means that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate; theterm “light valve” can also be used in this context. Generally, the saidpattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such a patterning device include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        piezoelectric actuation means. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning device can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from U.S. Pat.        Nos. 5,296,891 and 5,523,193, and PCT patent applications WO        98/38597 and WO 98/33096, which are incorporated herein by        reference. In the case of a programmable mirror array, the said        support structure may be embodied as a frame or table, for        example, which may be fixed or movable as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (e.g. silicon wafer)that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at one time; such an apparatus iscommonly referred to as a wafer stepper. In an alternativeapparatus—commonly referred to as a step-and-scan apparatus—each targetportion is irradiated by progressively scanning the mask pattern underthe projection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTpatent application WO 98/40791, incorporated herein by reference.

It has been proposed to immerse the substrate in a lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system.)

PCT patent application WO 99/49504 discloses a lithographic apparatus inwhich a liquid is supplied to the space between the projection lens andthe wafer. As the wafer is scanned beneath the lens in a −X direction,liquid is supplied at the +X side of the lens and taken up at the −Xside.

SUMMARY

Submersing the substrate table in liquid may mean that there is a largebody of liquid that must be accelerated during a scanning exposure. Thismay require additional or more powerful motors and turbulence in theliquid may lead to undesirable and unpredictable effects.

There are several difficulties associated with having liquids in alithographic projection apparatus. For example, escaping liquid maycause a problem by interfering with interferometers and, if thelithographic projection apparatus requires the beam to be held in avacuum, by destroying the vacuum. Furthermore, the liquid may be used upat a high rate unless suitable precautions are taken.

Further problems associated with immersion lithography may include thedifficulty in keeping the depth of the liquid constant and transfer ofsubstrates to and from the imaging position, i.e., under the finalprojection system element. Also, contamination of the liquid (bychemicals dissolving in it) and increase in temperature of the liquidmay deleteriously affect the imaging quality achievable.

In the event of a computer failure or power failure or loss of controlof the apparatus for any reason, steps may need to be taken to protect,in particular, the optical elements of the projection system. It may benecessary to take steps to avoid spillage of the liquid over othercomponents of the apparatus.

If a liquid supply system is used in which the liquid has a freesurface, steps may need to be taken to avoid the development of waves inthat free surface due to forces applied to the liquid supply system.Waves can transfer vibrations to the projection system from the movingsubstrate.

Accordingly, it may be advantageous to provide, for example, alithographic projection apparatus in which a space between the substrateand the projection system is filled with a liquid while minimizing thevolume of liquid that must be accelerated during stage movements.

According to an aspect, there is provided a lithographic projectionapparatus, comprising:

-   -   a support structure configured to hold a patterning device, the        patterning device configured to pattern a beam of radiation        according to a desired pattern;    -   a substrate table configured to hold a substrate;    -   a projection system configured to project the patterned beam        onto a target portion of the substrate; and    -   a liquid supply system configured to at least partly fill a        space between said projection system and said substrate, with a        liquid through which said beam is to be projected, said liquid        supply system comprising:    -   a liquid confinement structure extending along at least a part        of the boundary of said space between said projection system and        said substrate table, and    -   a gas seal between said structure and the surface of said        substrate.

A gas seal forms a non-contact seal between the structure and thesubstrate so that the liquid is substantially contained in the spacebetween the projection system and the substrate, even as the substratemoves under the projection system, e.g. during a scanning exposure.

The structure may be provided in the form of a closed loop, whethercircular, rectangular, or other shape, around the space or may beincomplete, e.g., forming a U-shape or even just extending along oneside of the space. If the structure is incomplete, it should bepositioned to confine the liquid as the substrate is scanned under theprojection system.

In an embodiment, the gas seal comprises a gas bearing configured tosupport said structure. This has an advantage that the same part of theliquid supply system can be used both to bear the structure and to sealliquid in a space between the projection system and the substrate,thereby reducing the complexity and weight of the structure. Also,previous experience gained in the use of gas bearings in vacuumenvironments can be called on.

In an embodiment, the gas seal comprises a gas inlet formed in a face ofsaid structure that opposes said substrate to supply gas and a first gasoutlet formed in a face of said structure that opposes said substrate toextract gas. Further, there may be provided a gas supply to provide gasunder pressure to said gas inlet and a vacuum device to extract gas fromsaid first gas outlet. In an embodiment, the gas inlet is locatedfurther outward from the optical axis of said projection system thansaid first gas outlet. In this way, the gas flow in the gas seal isinward and may most efficiently contain the liquid. In this case, thegas seal may further comprises a second gas outlet formed in the face ofthe structure which opposes the substrate, the first and second gasoutlets being formed on opposite sides of the gas inlet. The second gasoutlet helps to ensure minimal escape of gas from the gas inlet into anenvironment surrounding the structure. Thus, the risk of gas escapingand interfering with, for example, the interferometers or degrading avacuum in the lithographic apparatus, is minimized.

The liquid supply system may also comprise a sensor configured tomeasure the distance between the face of the structure and the substrateand/or the topography of the top surface of the substrate. In this way,controller can be used to vary the distance between the face of thestructure and the substrate by controlling, for example, the gas sealeither in a feed-forward or a feed-back manner.

The apparatus may further comprise a positioning device configured tovary the level of a portion of said face of said structure between thefirst gas outlet and an edge of the face nearest the optical axisrelative to the remainder of the face. This allows a pressure containingthe liquid in the space, to be controlled independently of the pressurebelow the inlet so that the height of the structure over the substratecan be adjusted without upsetting the balance of forces holding liquidin the space. An alternative way of ensuring this is to use apositioning device configured to vary the level of a portion of the facebetween the first or second gas outlets and the gas inlet relative tothe remainder of the face. Those three systems may be used in anycombination.

In an embodiment, there is provided a channel formed in the face of thestructure located nearer to the optical axis of the projection systemthan the first gas outlet. The pressure in that channel can be varied tocontain the liquid in the space whereas the gas in and out-lets may beused to vary the height of the structure above the substrate so thatthey only operate to support the structure and have little, if any,sealing function. In this way, it may possible to separate a sealingfunction and a bearing function of the gas seal.

In an embodiment, a porous member may be disposed over the gas inlet forevenly distributing gas flow over the area of the gas inlet.

In an embodiment, the gas in and out-lets may each comprise a groove insaid face of said structure opposing said substrate and a plurality ofconduits leading into said groove at spaced locations.

In an embodiment, the gap between said structure and the surface of saidsubstrate inwardly of said gas seal is small so that capillary actiondraws liquid into the gap and/or gas from the gas seal is prevented fromentering the space. The balance between the capillary forces drawingliquid under the structure and the gas flow pushing it out may form aparticularly stable seal.

In an embodiment, the liquid supply system is configured to at leastpartly fill a space between a final lens of the projection system andthe substrate, with liquid.

It may also be advantageous to provide, for example, a lithographicprojection apparatus in which a space between the substrate and theprojection system is filled with a liquid while minimizing atransmission of disturbance forces between the substrate and projectionsystem.

According to an aspect, there is provided a lithographic apparatus,comprising:

-   -   a support structure configured to hold a patterning device, the        patterning device configured to pattern a beam of radiation        according to a desired pattern;    -   a substrate table configured to hold a substrate;    -   a projection system configured to project the patterned beam        onto a target portion of the substrate; and    -   a liquid supply system configured to at least partly fill a        space between the final element of said projection system and        said substrate with a liquid, wherein said space is in liquid        connection with a liquid reservoir through a duct, and the        minimum cross sectional area of said duct in a plane        perpendicular to the direction of fluid flow is at least

${\pi \left( \frac{8\Delta \; V\; \eta \; L}{{\pi\Delta}\; P_{\max}t_{\min}} \right)}^{1\text{/}2},$

where ΔV is the volume of liquid which has to be removed from said spacewithin time t_(min), L is the length of the duct, η is viscosity ofliquid in said space and ΔP_(max) is the maximum allowable pressure onan element of said projection system.

Liquid may be completely constrained such that it does not have a largefree surface for the development of waves, i.e., the space or reservoiris enclosed at the top and the reservoir is full of liquid. This isbecause the amount of fluid which can flow through the duct in a giventime (time of crash measured experimentally) is large enough to avoiddamage to the final element of the projection system when the apparatuscrashes because the liquid can escape through the duct before pressurein the space builds up to levels at which damage may occur. The liquidescapes when the structure moves relative to the element otherwise thehydrostatic pressure applied to an element of the projection systemduring relative movement of the element to the structure may damage theelement.

According to an aspect, there is provided a lithographic apparatus,comprising:

-   -   a support structure configured to hold a patterning device, the        patterning device configured to pattern a beam of radiation        according to a desired pattern;    -   a substrate table configured to hold a substrate;    -   a projection system configured to project the patterned beam        onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between said projection system and said substrate with a        liquid, said liquid supply system comprising, on a top surface        of liquid in said liquid supply system, a wave suppression        device configured to suppress development of waves.

In this way, the development of waves can be suppressed by contact ofthe wave suppression device with a top surface of the liquid. In anembodiment, the wave suppression device comprises a pressure releasedevice. Thus, the liquid can escape from the space in the event of acrash to avoid damaging the element.

An example of a wave suppression device is a flexible membrane. In anembodiment, the wave suppression device may comprise placing a highviscosity liquid which is immiscible with the liquid in the space on thetop surface of the liquid in the space. In each of these cases, thepressure release functionality can be provided by the flexibility of thewave suppression device.

According to an aspect, there is provided a device manufacturing methodcomprising:

-   -   providing a liquid to a space between a projection system and a        substrate;    -   projecting a patterned beam of radiation, through said liquid,        onto a target portion of the substrate using the projection        system; and    -   forming a gas seal between a liquid confinement structure        extending along at least a part of the boundary of said space        and the surface of said substrate; or    -   providing a liquid reservoir in liquid connection with said        space through a duct and ensuring that said duct has a minimum        cross-sectional area in a plane perpendicular to the direction        of flow of liquid of

${\pi \left( \frac{8\Delta \; V\; \eta \; L}{{\pi\Delta}\; P_{\max}t_{\min}} \right)}^{1\text{/}2},$

where ΔV is the volume of liquid which has to be removed from said spacewithin time t_(min), L is the length of the duct, η is viscosity ofliquid in said space and ΔP_(max) is the maximum allowable pressure onan element of said projection system; or

-   -   suppressing development of waves on said liquid with a        suppression means and optionally, allowing for release of        pressure of said liquid.

Although specific reference may be made in this text to the use of theapparatus disclosed herein in the manufacture of ICs, it should beexplicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “reticle”, “wafer”or “die” in this text should be considered as being replaced by the moregeneral terms “mask”, “substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts the liquid reservoir of a first embodiment of theinvention;

FIG. 3 is an enlarged view of part of the liquid reservoir of the firstembodiment of the invention;

FIG. 4 depicts the liquid reservoir of a second embodiment of theinvention;

FIG. 5 is an enlarged view of part of the liquid reservoir of the secondembodiment of the invention;

FIG. 6 is an enlarged view of the liquid reservoir of a third embodimentof the present invention;

FIG. 7 depicts the liquid reservoir of a fourth embodiment of thepresent invention;

FIG. 8 is an enlarged view of part of the reservoir of the fourthembodiment of the present invention;

FIG. 9 depicts the liquid reservoir of a fifth embodiment of the presentinvention;

FIG. 10 depicts the liquid reservoir of a sixth embodiment of thepresent invention;

FIG. 11 depicts, in plan, the underside of the seal member of the sixthembodiment;

FIG. 12 depicts, in plan, the underside of the seal member of a seventhembodiment;

FIG. 13 depicts, in cross section, the liquid reservoir of the seventhembodiment;

FIG. 14 depicts, in cross section, the liquid reservoir of an eighthembodiment;

FIG. 15 depicts, in cross section, the liquid reservoir of a ninthembodiment;

FIG. 16 depicts, in cross section, the liquid reservoir of analternative ninth embodiment; and

FIG. 17 depicts, in cross section, the liquid reservoir of a tenthembodiment.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation (e.g. DUV radiation), which in this particular case        also comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning means for        accurately positioning the substrate with respect to item PL;    -   a projection system (“lens”) PL (e.g. a refractive lens system)        for imaging an irradiated portion of the mask MA onto a target        portion C (e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning means, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. an excimer laser) produces a beam of radiation. Thisbeam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed conditioning means, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (course positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

-   -   In step mode, the mask table MT is kept essentially stationary,        and an entire mask image is projected at one time (i.e. a single        “flash”) onto a target portion C. The substrate table WT is then        shifted in the x and/or y directions so that a different target        portion C can be irradiated by the beam PB;    -   In scan mode, essentially the same scenario applies, except that        a given target portion C is not exposed in a single “flash”.        Instead, the mask table MT is movable in a given direction (the        so-called “scan direction”, e.g. the y direction) with a speed        v, so that the projection beam PB is caused to scan over a mask        image; concurrently, the substrate table WT is simultaneously        moved in the same or opposite direction at a speed V=Mv, in        which M is the magnification of the lens PL (typically, M=¼ or        ⅕). In this manner, a relatively large target portion C can be        exposed, without having to compromise on resolution.

FIG. 2 shows a liquid reservoir 10 between the projection system PL anda substrate stage. The liquid reservoir 10 is filled with a liquid 11having a relatively high refractive index, e.g. water, provided viainlet/outlet ducts 13. The liquid has the effect that the radiation ofthe projection beam has a shorter wavelength in the liquid than in airor a vacuum, allowing smaller features to be resolved. It is well knownthat the resolution limit of a projection system is determined, interalia, by the wavelength of the projection beam and the numericalaperture of the system. The presence of the liquid may also be regardedas increasing the effective numerical aperture. Furthermore, at fixednumerical aperture, the liquid is effective to increase the depth offield.

The reservoir 10 forms a contactless seal to the substrate around theimage field of the projection system so that liquid is confined to filla space between the substrate W surface and the final element of theprojection system PL. The reservoir is formed by a seal member 12positioned below and surrounding the final element of the projectionsystem PL. Liquid is brought into the space below the projection systemPL and within the seal member 12. The seal member 12 extends a littleabove the final element of the projection system PL and the liquid levelrises above the final element so that a buffer of liquid is provided.The seal member 12 has an inner periphery that at the upper end, in anembodiment, closely conforms to the step of the projection system or thefinal element thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g.,rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the seal member 12 and the surface of the substrate W. The gasseal is formed by gas, e.g. air or synthetic air but in an embodiment,N₂ or another inert gas, provided under pressure via inlet 15 to the gapbetween seal member 12 and the substrate W and extracted via firstoutlet 14. The overpressure on the gas inlet 15, vacuum level on thefirst outlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow inwards that confines the liquid. This is shownin more detail in FIG. 3.

The gas seal is formed by two (annular) grooves 18, 19 which areconnected to the first inlet 15 and first outlet 14 respectively by aseries of small conducts spaced around the grooves. The in-and out-lets14, 15 may either be a plurality of discrete orifices around thecircumference of the seal member 12 or may be continuous grooves orslits. A large (annular) hollow in the seal member may be provided ineach of the inlet and outlet to form a manifold. The gas seal may alsobe effective to support the seal member 12 by behaving as a gas bearing.

Gap G1, on the outer side of the gas inlet 15, is, in an embodiment,small and long so as to provide resistance to gas flow outwards but neednot be. Gap G2, at the radius of the inlet 15, is a little larger toensure a sufficient distribution of gas around the seal member, theinlet 15 being formed by a number of small holes around the seal member.Gap G3 is chosen to control the gas flow through the seal. Gap G4 islarger to provide a good distribution of vacuum, the outlet 14 beingformed of a number of small holes in the same or similar manner as theinlet 15. Gap G5 is small to prevent gas/oxygen diffusion into theliquid in the space, to prevent a large volume of liquid entering anddisturbing the vacuum and to ensure that capillary action will alwaysfill it with liquid.

The gas seal is thus a balance between the capillary forces pullingliquid into the gap and the gas flow pushing liquid out. As the gapwidens from G5 to G4, the capillary forces decrease and the gas flowincreases so that the liquid boundary will lie in this region and bestable even as the substrate moves under the projection system PL.

The pressure difference between the inlet, at G2 and the outlet at G4 aswell as the size and geometry of gap G3, determine the gas flow throughthe seal 16 and will be determined according to the specific embodiment.However, a possible advantage is achieved if the length of gap G3 isshort and the absolute pressure at G2 is twice that at G4, in which casethe gas velocity will be the speed of sound in the gas and cannot riseany higher. A stable gas flow will therefore be achieved.

The gas outlet system can also be used to completely remove the liquidfrom the system by reducing the gas inlet pressure and allowing theliquid to enter gap G4 and be sucked out by the vacuum system, which caneasily be arranged to handle the liquid, as well as the gas used to formthe seal. Control of the pressure in the gas seal can also be used toensure a flow of liquid through gap G5 so that liquid in this gap thatis heated by friction as the substrate moves does not disturb thetemperature of the liquid in the space below the projection system.

The shape of the seal member around the gas inlet and outlet should bechosen to provide laminar flow as far as possible so as to reduceturbulence and vibration. Also, the gas flow should be arranged so thatthe change in flow direction at the liquid interface is as large aspossible to provide maximum force confining the liquid.

The liquid supply system circulates liquid in the reservoir 10 so thatfresh liquid is provided to the reservoir 10.

The gas seal 16 can produce a force large enough to support the sealmember 12. Indeed, it may be necessary to bias the seal member 12towards the substrate to make the effective weight supported by the sealmember 12 higher. The seal member 12 will in any case be held in the XYplane (perpendicular to the optical axis) in a substantially stationaryposition relative to and under the projection system but decoupled fromthe projection system. The seal member 12 is free to move in the Zdirection and Rx and Ry.

Embodiment 2

A second embodiment is illustrated in FIGS. 4 and 5 and is the same asthe first embodiment except as described below.

In this embodiment a second gas outlet 216 is provided on the oppositeside of the gas inlet 15 to the first gas outlet 14. In this way any gasescaping from the gas inlet 15 outwards away from the optical axis ofthe apparatus is sucked up by second gas outlet 216 which is connectedto a vacuum source 491. In this way gas is prevented from escaping fromthe gas seal so that it cannot interfere, for example, withinterferometer readings or with a vacuum in which the projection systemand/or substrate may be housed.

Another advantage of using the two gas outlet embodiment is that thedesign is very similar to that of gas bearings previously used inlithographic projection apparatus. Thus the experience gained with thosegas bearings can be applied directly to the gas seal of this embodiment.The gas seal of the second embodiment is particularly suitable for useas a gas bearing, as well as a seal means, such that it can be used tosupport the weight of the seal member 12.

Advantageously one or more sensors 492 may be provided to either measurethe distance between the bottom face of the seal member 12 and thesubstrate W or the topography of the top surface of the substrate W. Acontroller 493 may then be used to vary the pressures applied to the gasin- and out-lets 14, 15, 216 to vary the pressure P2 which constrainsthe liquid 11 in the reservoir and the pressures P1 and P3 which supportthe seal member 12. Thus the distance D between the seal member 12 andthe substrate W may be varied or kept at a constant distance. The samecontroller may be used to keep the seal member 12 level. The controllermay use either a feed forward or a feedback control loop.

FIG. 5 shows in detail how the gas seal can be regulated to controlindependently the pressure P2 holding the liquid 11 in the reservoir andP3 which supports the seal member 12. This extra control is advantageousbecause it provides a way of minimizing liquid losses during operation.The second embodiment allows pressures P2 and P3 to be controlledindependently to account for varying conditions during exposure. Varyingconditions might be different levels of liquid loss per unit timebecause of different scanning speeds or perhaps because the edge of asubstrate W is being overlapped by the seal member 12. This is achievedby providing means for varying the distance to the substrate W ofdiscrete portions of the face of the seal member 12 facing the substrateW. These portions include the portion 220 between the first gas outlet14 and the edge of the seal member 12 nearest the optical axis, theportion 230 between the gas inlet 15 and the first gas outlet 14 and theportion 240 between the second gas outlet 216 and the gas inlet 15.These portions may be moved towards and away from the substrate W by theuse of piezoelectric actuators for example. That is the bottom face 232of the seal member 12 may comprise piezoelectric actuators (e.g.,stacks) which can be expanded/contracted by the application of apotential difference across them. Other mechanical means could also beused.

The pressure P3 which is created below the gas inlet 15 is determined bythe pressure of gas P5 applied to the gas inlet 15, pressures of gas P6and P4 applied to the first and second gas outlets 14 and 216respectively and by the distance D between the substrate W and thebottom face of the seal member 12 facing the substrate W. Also thehorizontal distance between the gas in and out-lets has an effect.

The weight of the seal member 12 is compensated for by the pressure ofP3 so that the seal member 12 settles a distance D from the substrate W.A decrease in D leads to an increase in P3 and an increase in D willlead to a decrease in P3. Therefore this is a self regulating system.

Distance D, at a constant pushing force due to pressure P3, can only beregulated by pressures P4, P5 and P6. However, the combination of P5, P6and D creates pressure P2 which is the pressure keeping the liquid 11 inthe reservoir. The amount of liquid escaping from a liquid container atgiven levels of pressure can be calculated and the pressure in theliquid P_(LIQ) is also important. If P_(LIQ) is larger than P2, theliquid escapes from the reservoir and if P_(LIQ) is less than P2, gasbubbles will occur in the liquid which is undesirable. It is desirableto try to maintain P2 at a value slightly less than P_(LIQ) to ensurethat no bubbles form in the liquid but also to ensure that not too muchliquid escapes as this liquid needs to be replaced. In an embodiment,this can all be done with a constant D. If the distance D1 betweenportion 220 and the substrate W is varied, the amount of liquid escapingfrom the reservoir can be varied considerably as the amount of liquidescaping varies as a square of distance D1. The variation in distance isonly of the order of 1 mm, in an embodiment 10 μm and this can easily beprovided by a piezoelectric stack with an operational voltage of theorder of 100V or more.

Alternatively, the amount of liquid which can escape can be regulated byplacing a piezoelectric element at the bottom of portion 230. Changingthe distance D2 is effective to change pressure P2. However, thissolution might require adjustment of pressure P5 in gas inlet 15 inorder to keep D constant.

Of course the distance D3 between the lower part of portion 240 andsubstrate W can also be varied in a similar way and can be used toregulate independently P2 and P3. It will be appreciated that pressuresP4, P5 and P6 and distances D1, D2 and D3 can all be regulatedindependently or in combination to achieve the desired variation of P2and P3.

Indeed the second embodiment is particularly effective for use in activemanagement of the quantity of liquid in the reservoir 10. The standbysituation of the projection apparatus could be, where no substrate W isbeing imaged, that the reservoir 10 is empty of liquid but that the gasseal is active thereby to support the seal member 12. After thesubstrate W has been positioned, liquid is introduced into the reservoir10. The substrate W is then imaged. Before the substrate W is removed,the liquid from the reservoir can be removed. After exposure of the lastsubstrate the liquid in the reservoir 10 will be removed. Wheneverliquid is removed, a gas purge has to be applied to dry the areapreviously occupied by liquid. The liquid can obviously be removedeasily in the apparatus according to the second embodiment by variationof P2 while maintaining P3 constant as described above. In otherembodiments a similar effect can be achieved by varying P5 and P6 (andP4 if necessary or applicable).

Embodiment 3

As an alternative or a further development of the second embodiment asshown in FIG. 6, a channel 320 may be provided in the face of the sealmember 12 facing the substrate W inwardly (i.e. nearer to the opticalaxis of the projection system) of the first gas outlet 14. The channel320 may have the same construction as the gas in- and out-lets 14, 15,216.

Using the channel 320 pressure P2 may be varied independently ofpressure P3. Alternatively, by opening this channel to environmentalpressure above the liquid level in the reservoir 10, the consumption ofliquid from the reservoir during operation is greatly reduced. Thisembodiment has been illustrated in combination with the secondembodiment though the channel 320 may be used in combination with any ofthe other embodiments, in particular the first embodiment. A furtheradvantage is that the gas inlet 15 and first gas outlet 14 (and forcertain embodiments second gas outlet 216) are not disturbed.

Furthermore, although only three elements have been illustrated anynumber of channels may be incorporated into the face of the seal member12 facing the substrate W, each channel being at a pressure to improvestiffness, liquid consumption, stability or other property of the liquidsupply system.

Embodiment 4

A fourth embodiment which is illustrated in FIGS. 7 and 8 is the same asthe first embodiment except as described below. However, the fourthembodiment may also be advantageously used with any of the otherembodiments described.

In the fourth embodiment a porous member 410, in an embodiment porouscarbon or a porous ceramic member, is attached to the gas inlet 15 wheregas exits the bottom face of the seal member 12. In an embodiment, thebottom of the porous member is co-planar with the bottom of the sealmember. This porous carbon member 410 is insensitive to surfaces whichare not completely flat (in this case substrate W) and the gas exitingthe inlet 15 is well distributed over the entire exit of the inlet. Theadvantage gained by using the porous member 410 is also apparent whenthe seal member 12 is positioned partly over the edge of the substrate Was at this point the surface which the gas seal encounters is uneven.

In a variant of the fourth embodiment, the porous member 410 can beplaced in the vacuum channel(s) 14. The porous member 410 should have aporosity chosen to maintain under pressure while preventing unacceptablepressure loss. This is advantageous when imaging the edge of thesubstrate W and the gas bearing moves over the edge of the substrate Wbecause although the preload force at the position of the edge might belost, the vacuum channel is not contaminated with a large and variableamount of gas, greatly reducing variations in the preload and as aconsequence variation in flying height and forces on the stage.

Embodiment 5

All of the above described embodiments typically have liquid in thereservoir 10 exposed to a gas, such as air, with a free surface. This isto prevent the final element of the projection system PL from breakingin a case of a crash due to build up of hydrostatic forces on theprojection system. During a crash the liquid in the reservoir 10 isunconstrained such that the liquid will easily give, i.e. be forcedupwards, when the projection system PL moves against it. Thedisadvantage of this solution is that surface waves may occur on thefree surface during operation thereby transmitting disturbance forcesfrom the substrate W to the projection system PL, which is undesirable.

One way of solving this problem is to ensure that the reservoir 10 iscompletely contained within a seal member, particularly the uppersurface. Liquid is then fed to the reservoir 10 through a duct from asecondary reservoir. That secondary reservoir can have an unconstrainedtop surface and during a crash liquid is forced through the duct intothe second reservoir such that the build up of large hydrostatic forcesin the first reservoir 10 on the projection system can be avoided.

In such a closed system the local build up of pressure in the liquid onthe projection system is avoided by ensuring that the duct connectingthe reservoirs has a cross-sectional area equivalent to a duct with aradius according to the following equation

$R = \left( \frac{8\Delta \; V\; \eta \; L}{{\pi\Delta}\; {Pt}} \right)^{1\text{/}4}$

where R is the duct radius, ΔV is the volume of liquid which has to beremoved from the reservoir 10 within time t, L is the length of theduct, η is viscosity of the liquid and ΔP is the pressure differencebetween the secondary reservoir and the primary reservoir 10. If anassumption is made that the substrate table can crash with a speed of0.2 m/sec (measured by experiment) and ΔP_(max) is 10⁴ Pa (about themaximum pressure the final element of the project system can withstandbefore damage results), the pipe radius needed is about 2.5 millimetersfor a duct length of 0.2 m. In an embodiment, the effective radius ofthe duct is at least twice the minimum given by the formula.

An alternative way to avoid the buildup of waves in the liquid in thereservoir while still ensuring that the projection system PL isprotected in a crash, is to provide the free surface of the liquid witha suppression membrane 510 on the top surface of the liquid in thereservoir 10. This solution uses a safety means 515 to allow the liquidto escape in the case of a crash without the build-up of too high apressure. One solution is illustrated in FIG. 9. The suppressionmembrane may be made of a flexible material which is attached to thewall of the seal member 12 or the projection system in such a way thatbefore the pressure in the liquid reaches a predetermined allowedmaximum, liquid is allowed to deform the flexible suppression membrane510 such that liquid can escape between the projection system PL and thesuppression membrane 510 or between the suppression membrane and theseal member, respectively. Thus in a crash it is possible for liquid toescape above the safety membrane without damaging the projection systemPL. For this embodiment it is obviously desirable to have a space abovethe suppression membrane of at least the volume of a reservoir 10. Thusthe flexible membrane is stiff enough to prevent the formation of wavesin the top surface of the liquid in the reservoir 10 but is not stiffenough to prevent liquid escaping once the liquid reaches apredetermined hydrostatic pressure. The same effect can be achieved byuse of pressure valves 515 which allow the free-flow of liquid above apredetermined pressure in combination with a stiffer suppressionmembrane.

An alternative form of suppression means is to place a high viscosityliquid on the top free surface of the liquid in the reservoir 10. Thiswould suppress surface wave formation while allowing liquid to escapeout of the way of the projection system PL in the case of a crash.Obviously the high viscosity liquid must be immiscible with the liquidused in the space 10.

A further alternative for the liquid suppression means 510 is for it tocomprise a mesh. In this way the top surface of the liquid can be splitinto several parts each of smaller area. In this way, development oflarge surface waves which build up due to resonance and disturb theprojection system is avoided because the surface area of the severalparts is equal to the mesh opening so that the generation of largesurface waves is effectively damped. Also, as the mesh allows flow ofliquid through its openings, an effective pressure release mechanism isprovided for the protection of the projection system in the case of acrash.

Embodiment 6

A sixth embodiment as illustrated in FIGS. 10 and 11 is the same as thefirst embodiment except as described below. The sixth embodiment usesseveral of the ideas in the foregoing embodiments.

As with the other embodiments, the immersion liquid 11 is confined to anarea between the projection system PL and the substrate W by a sealmember 12, in this case, positioned below and surrounding the finalelement of the projection system PL.

The gas seal between the seal member 12 and the substrate W is formed bythree types of in-and-out-let. The seal member is generally made up ofan outlet 614, an inlet 615 and a further inlet 617. These arepositioned with the outlet 614 nearest the projection system PL, thefurther inlet 617 outwardly of the outlet 614 and the inlet 615 furthestfrom the projection system PL. The inlet 615 comprises a gas bearing inwhich gas is provided to a plurality of outlet holes 620 in the surfaceof the seal member 12 facing the substrate W via a (annular) chamber622. The force of the gas exiting the outlet 620 both supports at leastpart of the weight of the seal member 12 as well as providing a flow ofgas towards the outlet 614 which helps seal the immersion liquid to beconfined to a local area under the projection system PL. A purpose ofthe chamber 622 is so that the discrete gas supply orifice(s) 625provide gas at a uniform pressure at the outlet holes 620. The outletholes 620 are about 0.25 mm in diameter and there are approximately 54outlet holes 620. There is an order of magnitude difference in flowrestriction between the outlet holes 620 and the chamber 622 whichensures an even flow out of all of the outlet holes 620 despite theprovision of only a small number or even only one main supply orifice625.

The gas exiting the outlet holes 620 flows both radially inwardly andoutwardly. The gas flowing radially inwardly to and up the outlet 614 iseffective to form a seal between the seal member 12 and the substrate W.However, it has been found that the seal is improved if a further flowof gas is provided by a further inlet 617. Passage 630 is connected to agas source, for example the atmosphere. The flow of gas radiallyinwardly from the inlet 615 is effective to draw further gas from thefurther inlet 617 towards the outlet 614.

A (annular) groove 633 which is provided at the end of the passage 630(rather than a series of discrete inlets) ensures that the sealing flowof gas between the inner most edge of the groove 633 and the outlet 614is even around the whole circumference. The groove is typically 2.5 mmwide and of a similar height.

The inner most edge 635 of the groove 633 is, as illustrated, providedwith a radius to ensure smooth flow of the gas through passage 630towards the outlet 614.

The outlet 614 also has a continuous groove 640 which is approximatelyonly 0.7 mm high but 6 to 7 mm wide. The outer most edge 642 of thegroove 640 is provided as a sharp, substantially 90°, edge so that theflow of gas, in particular the flow of gas out of further inlet 617 isaccelerated to enhance the effectiveness of the gas seal. The groove 640has a plurality of outlet holes 645 which lead into a (annular) chamber647 and thus to discrete outlet passage 649. In an embodiment, theplurality of outlet holes 645 are approximately 1 mm in diameter suchthat liquid droplets passing through the outlet holes 645 are broken upinto smaller droplets.

The effectiveness of liquid removal of the seal member 12 can beadjusted by an adjustable valve 638 connected to the further inlet 617.The valve 638 is effective to adjust the flow through further inlet 617thereby to vary the effectiveness of liquid removal of the gas seal 12through outlet 614.

In an embodiment, the overall diameter of the seal member is of theorder of 100 mm.

FIG. 11 shows, in plan, the underside of the seal member 12 of FIG. 10.As can be seen, the inlet 615 is provided as a plurality of discreteinlet holes 620. This is advantageous over the use of a groove for themain inlet 615 because a groove as a gas bearing has a capacity (becauseof the compressible nature of gas) such that vibrations can be set up insuch a system. Small inlet holes 620 have a lower volume of gas in themand therefore suffer less from problems arising from capacity.

The use of a further inlet 617 in the form of a groove 633 can be usedto ensure a continuous gas flow around the whole periphery of the sealmember 12 which would not necessarily be possible when only usingdiscrete inlet holes 620. The provision of the outlets 645 as discreteentities is not a problem because of the provision of the groove 640which is effective, like chambers 647 and 622, to even out the flow.

The inlets for liquid are not illustrated in the seal member 12 of FIGS.10 and 11. The liquid may be provided in the same manner as illustratedin the foregoing embodiments or, alternatively, any of the liquid inletsand outlets as described in European patent application nos. EP03256820.6 and EP 03256809.9.

Embodiment 7

A seventh embodiment is similar to the sixth embodiment except asdescribed below. FIG. 12 is a plan view of the underside of the sealmember 12 similar to that shown in FIG. 11. In FIG. 12 the seal memberis not provided with a further inlet as in the sixth embodiment thoughthis can optionally be added. FIG. 13 shows a cross-section.

The seal member 12 of the seventh embodiment comprises a gas bearingformed by inlet holes 720 and which is of the same overall design as thesixth embodiment. An outlet 714 comprises a (annular) groove 740 withonly two passages 745, 747 which lead to a gas source and a vacuumsource respectively. In this way a high speed flow of gas from the gassource connected to passage 745 towards the vacuum source connected topassage 747 can be established. With this high speed flow of gas,immersion liquid may be drained more effectively. Furthermore, bycreating a larger restricted vacuum flow in the vacuum chamber, flowfluctuations due to variations in the height of the seal member 12 abovethe substrate W or other leakage sources in the surface will notinfluence the vacuum chamber pressure providing a preload for the gasbearing.

Embodiment 8

An eighth embodiment will be described in relation to FIG. 14 and is thesame as the first embodiment except as described below.

As can be seen from FIG. 14, the eighth embodiment has a seal member 12with an inlet 815 and an outlet 814 just like the first embodiment.However, a further inlet 817 is provided which is arranged so that a jetof gas can be formed which increases the velocity of the gas on thesurface of the substrate W below or radially outwardly of the outlet 814so that immersion liquid is more effectively removed from the surface ofthe substrate W. The further inlet 817 has an exit provided by a nozzlewhich is directed towards the substrate W at an angle radially inwardlytowards the projection system PL. Thus, the otherwise laminar gas flow(with a Reynolds number of around 300) between the inlet 815 and theoutlet 814 and which has a simple parabolic speed distribution with azero speed on the surface of the substrate, which may not be able toremove the last few micrometers of liquid film from the substrate, canbe improved because the further inlet 817 ensures that gas with a highergas velocity is in contact with the substrate surface.

From FIG. 14 it can be seen that the exit nozzle of the further inlet817 is provided radially outwardly of the outlet 814 but closer to theoutlet 814 than to the inlet 815.

Embodiment 9

A ninth embodiment is illustrated in FIGS. 15 and 16 and is the same asthe first embodiment except as described below.

In the ninth embodiment, the mouth of outlet 914 in the bottom surfaceof the seal member 12 which faces the substrate W, is modified toincrease the velocity of gas into the outlet 914. This is achieved byreducing the size of the mouth of the inlet 914 while keeping thepassageway of the outlet 914 the same size. This is achieved byproviding a smaller mouth by extending material of the seal member 12towards the center of the passage to form an outer additional member 950and an inner additional member 940. The outer additional member 950 issmaller than the inner additional member 940 and the gap between thosetwo members 940, 950 is, in an embodiment, approximately 20 timessmaller than the remainder of the outlet 914. In an embodiment, themouth is approximately 100 to 300 μm in width.

In FIG. 16 a further alternative version of the ninth embodiment isdepicted in which a further inlet 917 similar to the further inlet 817of the eight embodiment is provided. However, in this case the furtherinlet 917 provides a jet of flow substantially parallel to the surfaceof the substrate W so that the gas entering the mouth of the outlet 914is accelerated.

Embodiment 10

A tenth embodiment is illustrated in FIG. 17 and is the same as thefirst embodiment except as described below.

In the tenth embodiment, the efficiency of liquid removal may beimproved by increasing the velocity of gas on the surface of thesubstrate W along the same principles as in the eight embodiment. Gasleaving inlets 1015 and moving radially inwardly towards an outlet 1014passes underneath a (annular) groove 1018. The effect of the groove, asillustrated, is for the gas to enter the groove on its radially outermost side and to exit it, with an angle towards the substrate W, on theradially inward side. Thus, the speed of the gas on the surface of thesubstrate W at the entrance to the outlet 1014 is increased and liquidremoval efficiency is improved.

It will be clear that features of any embodiment can be used inconjunction with some or all features of any other embodiment.

In an aspect, there is provided a lithographic projection apparatus,comprising: a support structure configured to hold a patterning deviceand movable in a scanning direction, the patterning device configured topattern a beam of radiation according to a desired pattern; a substratetable configured to hold a substrate and movable in a scanningdirection; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid confinementstructure extending along at least a part of a boundary of a spacebetween the projection system and the substrate table, the space havinga cross-sectional area smaller than the area of the substrate, theliquid confinement structure positioned adjacent a final surface of theprojection system and comprising: a first inlet to supply liquid,through which the patterned beam is to be projected, to the space, afirst outlet to remove liquid after the liquid has passed under theprojection system, a second inlet formed in a face of the structure, theface arranged to oppose a surface of the substrate, and located radiallyoutward, with respect to an optical axis of the projection system, ofthe space to supply gas, and a second outlet formed in the face andlocated radially outward, with respect to an optical axis of theprojection system, of the second inlet to remove gas.

In an aspect, the area has a periphery conforming to a shape of an imagefield of the projection system. In an aspect, the inlet is configured tosupply the liquid at a first side of the projection system and theoutlet is configured to remove the liquid at a second side of theprojection system as the substrate is moved under the projection systemin a direction from the first side to the second side. In an aspect, thesecond inlet, the second outlet, or both, extend around the finalsurface of the projection system. In an aspect, the second inlet, thesecond outlet, or both, comprise a groove in the face. In an aspect, thegroove is circular and extends around the space. In an aspect, thestructure forms a closed loop around the space. In an aspect, theapparatus further comprises a porous member disposed over the secondinlet to evenly distribute gas flow over the area of the second inlet.

In an aspect, there is provided a lithographic projection apparatus,comprising: a support structure configured to hold a patterning device,the patterning device configured to pattern a beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid confinementstructure extending along at least a part of a boundary of a spacebetween the projection system and the substrate table, the space havinga cross-sectional area smaller than the area of the substrate, theliquid confinement structure positioned adjacent a final surface of theprojection system and comprising: a first inlet configured to supply aliquid, through which the patterned beam is to be projected, to thespace, a second inlet configured to supply gas and formed in a face ofthe structure, the face arranged to oppose a surface of the substrateand the second inlet located radially outward, with respect to anoptical axis of the projection system, of and substantially around thespace, and a second outlet configured to remove gas and formed in theface, the second outlet located radially outward, with respect to anoptical axis of the projection system, of and substantially around thesecond inlet.

In an aspect, the area has a periphery conforming to a shape of an imagefield of the projection system. In an aspect, the first inlet isconfigured to supply the liquid at a first side of the projection systemand the apparatus further comprises a second outlet configured to removethe liquid at a second side of the projection system as the substrate ismoved under the projection system in a direction from the first side tothe second side. In an aspect, the second inlet, the second outlet, orboth, comprise a groove in the face. In an aspect, the groove iscircular. In an aspect, the support structure and the substrate tableare movable in a scanning direction to expose the substrate. In anaspect, the apparatus further comprises a porous member disposed overthe second inlet to evenly distribute gas flow over the area of thesecond inlet.

In an aspect, there is provided a lithographic projection apparatus,comprising: a support structure configured to hold a patterning device,the patterning device configured to pattern a beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid confinementstructure extending along at least a part of a boundary of a spacebetween the projection system and the substrate table, the space havinga cross-sectional area smaller than the area of the substrate, theliquid confinement structure positioned adjacent a final surface of theprojection system and comprising: a first inlet configured to supply aliquid, through which the patterned beam is to be projected, to thespace, a second inlet configured to supply gas and formed in a face ofthe structure, the face arranged to oppose a surface of the substrateand the second inlet located radially outward, with respect to anoptical axis of the projection system, of the space and having a porousmember to evenly distribute gas flow over an area of the second inlet.

In an aspect, the apparatus further comprises a second outlet configuredto remove gas and formed in the face, the second outlet located radiallyoutward, with respect to an optical axis of the projection system, ofthe second inlet. In an aspect, the first inlet is configured to supplythe liquid at a first side of the projection system and the apparatusfurther comprises a second outlet configured to remove the liquid at asecond side of the projection system as the substrate is moved under theprojection system in a direction from the first side to the second side.In an aspect, the support structure and the substrate table are movablein a scanning direction to expose the substrate. In an aspect, thestructure forms a closed loop around the space.

In an aspect, there is provided a lithographic projection apparatuscomprising: a support structure configured to hold a patterning device,the patterning device configured to pattern a beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid supply systemconfigured to at least partly fill a space between said projectionsystem and said substrate, with a liquid through which said beam is tobe projected, said liquid supply system comprising: a liquid confinementstructure extending along at least a part of the boundary of said space,and a seal between said structure and the surface of said substrate.

In an aspect, said seal comprises a gas inlet formed in a face of saidstructure that opposes said substrate to supply gas and a gas outletformed in a face of said structure that opposes said substrate toextract gas. In an aspect, said gas seal comprises a gas supply toprovide gas under pressure to said gas inlet and a vacuum device toextract gas from said gas outlet. In an aspect, said gas inlet islocated radially further from the optical axis of said projection systemthan is said gas outlet. In an aspect, said gas inlet and said gasoutlet each comprise a groove in said face of said structure opposingsaid substrate and a plurality of conduits leading into said groove atspaced locations. In an aspect, said gas inlet and said gas outlet eachcomprises a manifold between said conduits and a gas source and a vacuumpump respectively. In an aspect, the gap between said structure and thesurface of said substrate inwardly of said gas seal is small so thatcapillary action at least one of draws liquid into the gap and preventsgas from said gas seal entering said space. In an aspect, said structureforms a closed loop around said space between said projection system andsaid substrate. In an aspect, said structure has an inner peripheryclosely conforming to the shape of the image field of said projectionsystem. In an aspect, said substrate table further comprises a coverplate surrounding said substrate, in use, and having an upper surfacesubstantially coplanar therewith. In an aspect, the apparatus furthercomprises a controller configured to control the gas pressure in saidgas seal to control the stiffness between said structure and saidsubstrate. In an aspect, said structure is stationary relative to saidprojection system. In an aspect, said support structure and saidsubstrate table are movable in a scanning direction to expose saidsubstrate. In an aspect, said liquid supply system comprises at leastone inlet to supply said liquid onto the substrate and at least oneoutlet to remove said liquid after said liquid has passed under saidprojection system. In an aspect, said liquid supply system is configuredto at least partly fill a space between a final lens of said projectionsystem and said substrate, with said liquid.

In an aspect, there is provided an immersion lithographic projectionapparatus, comprising: a support structure configured to hold apatterning device, the patterning device configured to pattern a beamaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid confinementstructure extending along at least part of the boundary of a spacebounded by the periphery of a localized area on the surface of saidsubstrate and said surface of said substrate, said space configured tocontain a liquid through which said beam is projected and said structuresubstantially sealing at least part of said space.

In an aspect, said support structure and said substrate table aremovable in a scanning direction to expose said substrate. In an aspect,said structure comprises at least one inlet to supply said liquid ontothe substrate and at least one outlet to remove said liquid after saidliquid has passed under said projection system. In an aspect, theapparatus comprises a gas seal between said structure and saidsubstrate. In an aspect, said gas seal comprises a gas inlet formed in aface of said structure that opposes said substrate to supply gas and agas outlet formed in a face of said structure that opposes saidsubstrate to extract gas.

In an aspect, there is provided a device manufacturing methodcomprising: providing a liquid to fill a space between a substrate and aprojection system, a liquid confinement structure extending along atleast a part of the boundary of said space; forming a gas seal betweensaid structure and the surface of said substrate; and projecting apatterned beam of radiation, through said liquid, onto a target portionof the substrate.

In an aspect, forming said gas seal comprises supplying a gas through agas inlet formed in a face of said structure that opposes said substrateand extracting gas through a gas outlet formed in a face of saidstructure that opposes said substrate. In an aspect, the methodcomprises supplying said gas at a position radially further from theoptical axis of said projection system than said extracting of gas. Inan aspect, the method comprises maintaining the gap between saidstructure and the surface of said substrate inwardly of said gas sealsmall so that capillary action at least one of draws liquid into the gapand prevents gas from said gas seal entering said space. In an aspect,said structure forms a closed loop around said space between saidprojection system and said substrate. In an aspect, said structure hasan inner periphery closely conforming to the shape of the image field ofsaid projection system. In an aspect, the method further comprisescontrolling the gas pressure in said gas seal to control the stiffnessbetween said structure and said substrate. In an aspect, the methodcomprises moving said support structure and said substrate table in ascanning direction to expose said substrate. In an aspect, the methodcomprises supplying said liquid onto the substrate and removing saidliquid after said liquid has passed under said projection system. In anaspect, providing a liquid to fill comprises providing a liquid to filla space between a substrate and a final lens of said projection system.

In an aspect, there is provided an immersion lithographic projectionapparatus comprising: a support structure configured to hold apatterning device and movable in a scanning direction, the patterningdevice configured to pattern a beam of radiation according to a desiredpattern; a substrate table configured to hold a substrate and movable ina scanning direction; a projection system configured to project thepatterned beam onto a target portion of the substrate using a scanningexposure; a liquid confinement structure that substantially seals atleast part of a space bounded by a surface of said substrate and theboundary of a portion of said surface; and a liquid inlet to provide aliquid, through which said beam is projected, to said space.

In an aspect, said inlet is configured to supply said liquid onto thesubstrate and comprising an outlet to remove said liquid after saidliquid has passed under said projection system. In an aspect, theapparatus comprises a gas seal between said structure and said surfaceof said substrate. In an aspect, said gas seal comprises a gas inletformed in a face of said structure that opposes said surface to supplygas and a gas outlet formed in a face of said structure that opposessaid surface to extract gas. In an aspect, said boundary conforms to ashape of an image field of said projection system.

In an aspect, there is provided a lithographic projection apparatuscomprising: a support structure configured to hold a patterning deviceand movable in a scanning direction, the patterning device configured topattern a beam of radiation according to a desired pattern; a substratetable configured to hold a substrate and movable in a scanningdirection; a projection system configured to project the patterned beamonto a target portion of the substrate using a scanning exposure; aliquid confinement structure having an aperture having a cross-sectionalarea smaller than a surface area of said substrate; a seal between saidstructure and said substrate; and a liquid inlet to provide a liquid,through which said beam is projected, to said aperture.

In an aspect, said inlet is configured to supply said liquid onto thesubstrate and comprising an outlet to remove said liquid after saidliquid has passed under said projection system. In an aspect, said sealis a gas seal. In an aspect, said gas seal comprises a gas inlet formedin a face of said structure that opposes a surface of said substrate tosupply gas and a gas outlet formed in a face of said structure thatopposes said surface of said substrate to extract gas. In an aspect,said aperture has a periphery conforming to a shape of an image field ofsaid projection system.

In an aspect, there is provided an immersion lithographic projectionapparatus comprising: a support structure configured to hold apatterning device, the patterning device configured to pattern a beam ofradiation according to a desired pattern; a substrate table configuredto hold a substrate; a projection system configured to project thepatterned beam onto a target portion of the substrate; a liquidconfinement structure that can substantially confine all of a liquidprovided to an area of a radiation-sensitive surface of said substrateunder said projection system, said area being smaller than the entirearea of said substrate surface; and a liquid inlet to provide a liquidto said area and between said projection system and said substratesurface.

In an aspect, said support structure and said substrate table aremovable in a scanning direction to expose said substrate. In an aspect,said inlet is configured to supply said liquid to said substrate surfaceand comprising an outlet to remove said liquid after said liquid haspassed under said projection system. In an aspect, said inlet suppliessaid liquid at a first side of said projection system and said outletremoves said liquid at a second side of said projection system as saidsubstrate is moved under said projection system in a direction from thefirst side to the second side. In an aspect, said structure comprises agas seal. In an aspect, said gas seal comprises a gas inlet formed in aface of said structure that opposes said substrate surface and a gasoutlet formed in a face of said structure that opposes said substratesurface. In an aspect, said area has a periphery conforming to a shapeof an image field of said projection system.

In an aspect, there is provided a lithographic projection apparatuscomprising: a support structure configured to hold a patterning device,the patterning device configured to pattern a beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid supply systemconfigured to at least partly fill a space between said projectionsystem and said substrate, with a liquid through which said beam is tobe projected, said liquid supply system comprising: a liquid confinementstructure extending along at least a part of the boundary of said spacebetween said projection system and said substrate table, and a gas sealbetween said structure and the surface of said substrate.

In an aspect, said gas seal comprises a gas bearing configured tosupport said structure over said substrate. In an aspect, said gas sealcomprises a gas inlet formed in a face of said structure that opposessaid substrate to supply gas and a first gas outlet formed in a face ofsaid structure that opposes said substrate to extract gas. In an aspect,said gas seal comprises a gas supply to provide gas under pressure tosaid gas inlet and a vacuum device to extract gas from said first gasoutlet. In an aspect, the apparatus further comprises a further inletconnected to a gas source and positioned between said first gas outletand said gas inlet. In an aspect, said further inlet comprises acontinuous annular groove in a surface of said structure facing saidsubstrate. In an aspect, a radially innermost corner of said groove hasa radius. In an aspect, said first gas outlet comprises a continuousannular groove in a surface of said structure facing said substrate. Inan aspect, at least one of said first gas outlet and said gas inletcomprise a chamber between a gas supply and a vacuum device respectivelyand a respective opening of said at least one of said first gas outletand said gas inlet in said surface, wherein said chamber provides alower flow restriction than said opening. In an aspect, said gas inletcomprises a series of discrete openings in a surface of said structurefacing said substrate. In an aspect, said first gas outlet comprises agroove in said face of said structure opposing said substrate, a firstpassage in said groove connected to a vacuum source and a second passagein said groove connected to a gas supply. In an aspect, a porous memberis disposed over said gas inlet to evenly distribute gas flow over thearea of said gas inlet. In an aspect, a porous member is disposed oversaid first gas outlet to evenly distribute gas flow over the area ofsaid first gas outlet. In an aspect, said structure further comprises asecond gas outlet formed in said face of said structure that opposessaid substrate, said first and second gas outlets being formed onopposite sides of said gas inlet. In an aspect, the apparatus furthercomprises a positioning device configured to vary the level of a portionof said face between said second gas outlet and said gas inlet relativeto the remainder of said face. In an aspect, the apparatus furthercomprises a positioning device configured to vary the level of a portionof said face between said first gas outlet and said gas inlet relativeto the remainder of said face. In an aspect, the apparatus furthercomprises a positioning device configured to vary the level of a portionof said face between said first gas outlet and an edge of said facenearest said optical axis relative to the remainder of said face. In anaspect, said gas seal comprises a channel formed in said face andlocated nearer to the optical axis of the projection system than saidfirst gas outlet. In an aspect, said channel is a second gas inlet. Inan aspect, said channel is open to the environment above the level ofliquid in said space. In an aspect, said gas inlet is located furtheroutward from the optical axis of said projection system than is saidfirst gas outlet. In an aspect, said gas inlet and said first gas outleteach comprise a groove in said face of said structure opposing saidsubstrate and a plurality of conduits leading into said groove at spacedlocations. In an aspect, the apparatus further comprises a sensorconfigured to measure the distance between said face of said structureand at least one of said substrate and the topography of said substrate.In an aspect, the apparatus further comprises a controller configured tocontrol the gas pressure in said gas seal to control at least one of thestiffness between said structure and said substrate and the distancebetween said structure and said substrate. In an aspect, the gap betweensaid structure and the surface of said substrate inwardly of said gasseal is small so that capillary action at least one of draws liquid intothe gap and reduces gas from said gas seal entering said space. In anaspect, said structure forms a closed loop around said space betweensaid projection system and said substrate. In an aspect, the apparatuscomprises on a top surface of liquid in said liquid supply system, awave suppression device configured to suppress development of waves. Inan aspect, said wave suppression device comprises a pressure releasedevice. In an aspect, the apparatus comprises a further gas inlet formedin a face of said structure that opposes said substrate, disposedbetween said first gas outlet and said gas inlet and angled radiallyinwardly towards an optical axis of the projection system to provide ajet of gas. In an aspect, the apparatus comprises a groove formed in aface of said structure that opposes said substrate and disposed betweensaid first gas outlet and said gas inlet. In an aspect, said liquidsupply system comprises at least one inlet to supply said liquid ontothe substrate and at least one outlet to remove said liquid after saidliquid has passed under said projection system. In an aspect, saidsupport structure and said substrate table are movable in a scanningdirection to expose said substrate. In an aspect, said liquid supplysystem is configured to at least partly fill a space between a finallens of said projection system and said substrate, with said liquid.

In an aspect, there is provided a lithographic projection apparatuscomprising: a support structure configured to hold a patterning device,the patterning device configured to pattern a beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a liquid supply systemconfigured to at least partly fill a space between said projectionsystem and said substrate with a liquid, wherein said space is in liquidconnection with a liquid reservoir through a duct, and the minimum crosssectional area of said duct in a plane perpendicular to the direction offluid flow is at least

${\pi \left( \frac{8\Delta \; V\; \eta \; L}{{\pi\Delta}\; P_{\max}t_{\min}} \right)}^{1\text{/}2}$

where ΔV is the volume of liquid which has to be removed from said spacewithin time t_(min), L is the length of the duct, η is viscosity ofliquid in said space and ΔP_(max) is the maximum allowable pressure onan element of said projection system.

In an aspect, said space is enclosed such that when liquid is present insaid space, said liquid has no free upper surface.

In an aspect, there is provided a lithographic projection apparatuscomprising: a support structure configured to hold a patterning device,the patterning device configured to pattern a beam of radiationaccording to a desired pattern; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; a liquid supply systemconfigured to at least partly fill a space between said projectionsystem and said substrate with a liquid, said liquid supply systemcomprising on a top surface of liquid in said liquid supply system, awave suppression device configured to suppress development of waves.

In an aspect, said wave suppression device comprises a flexiblemembrane. In an aspect, said wave suppression device comprises a meshsuch that the maximum area of said top surface of said liquid is equalto the mesh opening. In an aspect, said wave suppression devicecomprises a high viscosity liquid which is immiscible with said liquid.In an aspect, said wave suppression device comprises a pressure releasedevice. In an aspect, said pressure release device comprises a safetyvalve configured to allow the passage therethrough of liquid above acertain pressure.

In an aspect, there is provided a lithographic projection apparatuscomprising: a support structure configured to hold a patterning deviceand movable in a scanning direction, the patterning device configured topattern a beam of radiation according to a desired pattern; a substratetable configured to hold a substrate and movable in a scanningdirection; a projection system configured to project the patterned beamonto a target portion of the substrate using a scanning exposure; and aliquid supply system configured provide a liquid, through which saidbeam is to be projected, to a space between said projection system andsaid substrate, said liquid supply system comprising: a liquidconfinement structure extending along at least a part of the boundary ofsaid space between said projection system and said substrate table, agas inlet formed in a face of said structure that opposes said substrateto supply gas, a gas outlet formed in a face of said structure thatopposes said substrate to extract gas, an inlet to supply said liquid tosaid substrate, and an outlet to remove said liquid after said liquidhas passed under said projection system.

In an aspect, said liquid supply system provides liquid to only alocalized area of said substrate. In an aspect, said area has aperiphery conforming to a shape of an image field of said projectionsystem. In an aspect, said inlet supplies said liquid at a first side ofsaid projection system and said outlet removes said liquid at a secondside of said projection system as said substrate is moved under saidprojection system in a direction from the first side to the second side.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1.-112. (canceled)
 113. A liquid confinement structure for alithographic apparatus, the liquid confinement structure configured toextend along at least a part of a boundary of a space between aprojection system of the lithographic apparatus and a movable table ofthe lithographic apparatus and the liquid confinement structurecomprising: a body defining an aperture arranged to allow a beam ofradiation of the projection system to pass therethrough to the space; afirst inlet arranged to supply liquid to the space; an outlet arrangedto remove liquid from the space, an opening of the outlet locatedoutward of the aperture; and a second inlet arranged to supply gas, anopening of the second inlet located outward of the aperture and of theopening of the outlet.
 114. The liquid confinement structure of claim113, wherein the aperture has a periphery conforming to a shape of aperiphery of the projection system.
 115. The liquid confinementstructure of claim 113, wherein the aperture has a cross-sectional areasmaller than the area of a substrate onto which the projection system isconfigured to project the radiation.
 116. The liquid confinementstructure of claim 113, wherein the opening of the outlet and theopening of the second inlet are located in a face of the liquidconfinement structure, the face arranged to oppose, when installed inthe lithographic apparatus, the movable table.
 117. The liquidconfinement structure of claim 113, wherein the liquid confinementstructure is configured to form a closed loop around the space and tosurround a final optical element of the projection system.
 118. Theliquid confinement structure of claim 113, wherein one or more openingsof the first inlet are located on opposite sides of a path of theradiation through the space.
 119. The liquid confinement structure ofclaim 113, wherein the face comprises a groove enclosing the aperture.120. The liquid confinement structure of claim 113, further comprising afurther outlet arranged to remove fluid from the space, an opening ofthe further outlet located outward of the aperture, of the outlet, andof the second inlet.
 121. A liquid confinement structure for alithographic apparatus, the liquid confinement structure configured toextend along at least a part of a boundary of a space between aprojection system of the lithographic apparatus and a table of thelithograph apparatus movable with respect to the liquid confinementstructure and the liquid confinement structure comprising: a bodydefining an aperture in a face of the liquid confinement structure, theface arranged to oppose, when installed in the lithographic apparatus,the movable table and the aperture arranged to allow a beam of radiationof the projection system to pass therethrough to the space; an outletarranged to remove liquid from the space, an opening of the outletlocated in the face and located outward of the aperture; and an inletarranged to supply gas, an opening of the inlet located in the face andlocated outward of the aperture and of the opening of the outlet. 122.The liquid confinement structure of claim 121, wherein the aperture hasa cross-sectional area smaller than the area of a substrate onto whichthe projection system is configured to project the radiation.
 123. Theliquid confinement structure of claim 121, wherein the liquidconfinement structure is configured to form a closed loop around thespace and to surround a final optical element of the projection system.124. The liquid confinement structure of claim 121, further comprising afurther outlet arranged to remove fluid from the space, an opening ofthe further outlet located outward of the aperture, of the outlet, andof the inlet.
 125. The liquid confinement structure of claim 121,wherein the face comprises a groove enclosing the aperture.
 126. Aliquid confinement structure for a lithographic apparatus, the liquidconfinement structure configured to extend along at least a part of aboundary of a space between a projection system of the lithographicapparatus and a table of the lithograph apparatus movable with respectto the liquid confinement structure and the liquid confinement structurecomprising: an aperture through which a beam from the projection systemcan pass and through which the liquid can pass; and an inlet arranged tosupply liquid to the space above the aperture; and an outlet arranged toremove liquid, supplied by the inlet, from the space below the aperture.127. The liquid confinement structure of claim 126, wherein the aperturehas a periphery conforming to a shape of an image field of theprojection system.
 128. The liquid confinement structure of claim 126,wherein the liquid confinement structure is configured to form a closedloop around the space and to enclose a final optical element of theprojection system.
 129. The liquid confinement structure of claim 126,wherein an opening of the outlet is located in a face of the liquidconfinement structure, the face arranged to oppose, when installed inthe lithographic apparatus, the movable table, and the opening of theoutlet is located outward of the aperture.
 130. The liquid confinementstructure of claim 126, further comprising an opening located outward ofan opening of the outlet, the opening arranged to supply a gas.
 131. Theliquid confinement structure of claim 126, wherein at least part of theliquid confinement structure is sized and arranged to extend to within aboundary of a surface of a final optical element of the projectionsystem.
 132. The liquid confinement structure of claim 126, wherein oneor more openings of the inlet are located on opposite sides of a path ofthe radiation through the space.